Stacked Layers of Dendrites and Needles Linked with Supercooled Cloud Water in Winter Storms

Well-known results from laboratory diffusion chambers (e.g., Bailey and Hallett, 2009) have shown the prevalence of needle/column crystal shapes in a temperature range of -4 to -7 ^oC and dendritic crystal shapes in a range from -10 to -15 ^oC, both under conditions of controlled water saturation. In situ measurements of winter storms with the NRC Convair-580 in the northeastern United States and southeastern Canada, aimed at both identification and the dual-polarimetric radar validation of aircraft icing hazard, have shown remarkable prevalence for the same temperature/shape scenario in natural conditions of large-scale ascent. An aircraft ‘porpoising’ maneuver over long horizontal tracks with the Convair-580 is well suited to sampling the needle layer at low altitude, the dendrite layer at higher altitude and a layer of supercooled water with proximity to both crystal layers. Comparisons between the in situ measurements and the ground-based dual-polarimetric radar observations have shown evidence for positive ZDR enhancements (1-3 dB) in each of two crystal layers, and with more isotropic response associated with the layer of maximum liquid water content where small lump graupel and/or drizzle are sometimes present. Examples will be shown from the BAIRS (Buffalo Area Icing and Radar Study) I field campaign in February 2013 and from the more recent BAIRS II campaign in winter 2017.